Conventional systems to monitor and control the flow of liquid agricultural agents at low flow rates contemplate a pump, a length of relatively large-diameter tubing to transport the liquid from a reservoir tank to a dispensing location, a sensing and control means, and a nozzle to restrict the flow of the liquid and to provide a controlled rate of application of the liquid.
The typical application for such a system is to dispense the liquid agricultural agents which may include insecticides, herbicides, fungicides, and/or fertilizers to crops. In one typical application, a farm tractor carries a tank of concentrated liquid and is driven across a field of row crops. A boom is attached to the tractor perpendicular to the direction of travel. The boom generally has one or more nozzles which are directed downward and spaced along the boom to dispense the liquid between the rows.
In another typical application, the liquid dispensers are attached to a mower deck in order to save labor by allowing simultaneous mowing and spraying of the liquid agricultural agent.
A nozzle in such a system comprises a relatively small aperture which restricts the flow of the liquid, and is typically located at the terminus of the tubing. The pump in such a system causes the liquid to flow through the tubing, and typically has the capacity to cause much more liquid to be dispensed than is desired. By restricting the flow of the liquid, the nozzle thus provides a means to regulate the amount of liquid dispensed. There is a major drawback in using a nozzle, however, in that a nozzle having a small enough aperture to provide a small flow rate tends to clog, either with particles suspended in the liquid, or with debris from the external environment (e.g., blown dirt and grass clippings from a mower). If a strainer having small enough openings to block particles that would clog the nozzle is used upstream from the nozzle, the strainer tends to clog.
Since the nozzles tend to wear (i.e., where the aperture becomes larger) or partially clog (i.e., where the aperture becomes smaller), it is difficult to maintain accurate dispensing rates at small flow rates. In systems with multiple nozzles, it also tends to be difficult to achieve accurate and equal dispensing rates through each of the several nozzles.
If a nozzle "blows out" or is accidentally knocked off by, for example, a collision with overhanging tree branches, it is difficult to prevent a massive and potentially dangerous "dumping" of the potentially hazardous liquid chemical in one place. The pump in these systems typically has the capacity to rapidly empty the chemical tank. Such accidents can be very expensive to clean up; the soil may even have to be dug up and burned to purge the spilt chemical.
Another method uses a separate metering pump on each dispensing line, however metering pumps are expensive, and generally can provide only open-loop control which tends to be inaccurate.
The term "liquid" as used in this discussion is meant to be inclusive of solutions, emulsions, suspensions and the like.
U.S. Pat. No. 4,433,811 by Godfrey et al. teaches use of a control means which includes a capillary tube taking fluid from a manifold and returning that fluid to a supply tank in an apparatus which controls the rate of application of liquid dyes to a moving textile material.
U.S. Pat. No. 5,260,875 by Tofte et al. and assigned to the assignee of the present invention, which is hereby incorporated by reference, teaches a distributed controller system which uses a half-duplex serial line to communicate between all controllers in a planting and spraying system.
What is needed is a system which can accurately dispense a liquid agricultural agent having possible particles without undue clogging at low flow rates.